1. Field of the Invention
This invention relates to a semiconductor device suitable for a ferroelectric memory and a method for manufacturing the same.
2. Description of the Related Art
A nonvolatile memory is a semiconductor memory which can store data even when a power supply is turned off. As a kind of the nonvolatile memory, there exists a ferroelectric memory using ferroelectric materials for a capacity insulating film.
The ferroelectric memory utilizes characteristics of two remanent polarizations whose polarities are different from each other, so that data can be stored if the power supply is turns off. The ferroelectric memory endures, in comparison with the other nonvolatile memories, a large number of write/erase cycles giving an indication of nonvolatility, about 1010 times to 1012 times. Write/erase speed thereof is also high, on the order of several dozen nanoseconds.
In the ferroelectric memory, a ferroelectric material can be polarized in two opposite directions. If being polarized in one direction is stored as information of “1”, and being polarized in the other direction is stored as information of “0”, the stored information can be discriminated by distinguishing polarization directions. A polarization direction of a ferroelectric material can be detected by giving the ferroelectric material enough electric potential to reverse the polarization.
In a case that the capacity insulating film is made of a dielectric material, not of the ferroelectric material, the polarization maintains only while a potential difference exists between electrodes. And when the potential difference is removed, the polarization vanishes. Therefore, information is not maintained.
As the ferroelectric materials, lead-system ferroelectric materials and bismuth-system ferroelectric materials can be cited. As the lead-system ferroelectric materials, PZT (PbZrxTi1-xO3) PLZT (PbyLa1-yZrxTi1-xO3) and the like can be cited. As the bismuth-system ferroelectric materials, SBT (SrBi2Ta2O9), BIT (Bi4Ti3O12) and the like can be cited.
Because a system LSI using the ferroelectric capacitor may be applied to the equipment in which money information and personal information stored in an IC card or a smart card and the like are handled, extremely high reliability is required. In addition, the system LSI requires a memory retention period for 10 years, write/erase cycles of 1015 times, and a guarantee for operational stability at the temperature of −45° C. to +125° C.
FIG. 8 is a cross-sectional view showing a part of a conventional ferroelectric memory. An interlayer insulating film 104 is formed over a semiconductor substrate 101, and a ferroelectric capacitor 105 is formed thereon. The ferroelectric capacitor is provided with a lower electrode 106 made of a Pt film, a capacity insulating film 107 made of a PZT film, and an upper electrode 108 made of the Pt film. On a surface of the semiconductor substrate 101, an element isolation insulating film 102 and a transistor 103 are formed. In the interlayer insulating film 104, a contact plug 109 is embedded. The contact plug 104 reaches to a diffusion layer of the transistor 103. An insulating film 110 covering the ferroelectric capacitor 105 is formed on the interlayer insulating film 104. A contact hole reaching to the upper electrode 108 is formed in the insulating film 110. And an Al wiring 111 which connects the contact plug 109 to the upper electrode 108 is formed.
In the conventional ferroelectric memory thus fabricated, as one of the way to increase a capacity of the ferroelectric capacitor, an enlargement of an area of the capacity insulating film 107 is considered, however, it will lead to the enlargement of a chip area directly. Meanwhile, with a miniaturization technique progresses, there arises a situation that the capacity has to be lowered. Besides, higher electric charge amount of a remanent polarization is also required.
Prior arts are disclosed in Japanese Patent No. 3435966 and Japanese Patent Application Laid-open No. Hei 06-021338.